Unicompartmental replacement using the Marmor prosthesis was done in 40 knees (37 patients) with medial compartment gonarthrosis. Of the 27 available for follow-up, results were considered excellent in 18 cases and good in seven, while two cases were considered failures. Meticulous removal of osteophytes by intercondylar burring and accurate alignment of the femoral component of the tibial medial compartment with the axis of the normal medial femoral condyle are essential to success. Preservation of a good articular surface and early mobilization with excellent range of motion are especially beneficial to elderly patients. Early results of this preliminary report indicate that unicompartmental replacement may be useful in the treatment of medial gonarthrosis.